1. Technical Field
The present invention relates to oxide powders of materials with a high electric permittivity and a manufacturing method thereof.
2. Discussion of the Related Art
Dielectric oxides are likely to have many applications due to their high theoretical electric permittivity, for example, from 9,000 to 12,000 for ceramics of type Ca0.25Cu0.75TiO3. In microelectronics, such dielectric ceramics are likely to be used as insulators in the manufacturing of integrated circuits. To deposit such a ceramic in the form of a thin layer with a thickness on the order of from 100 to 500 nm, it has been provided to use methods of bombarding a target formed by the ceramic.
The wet synthesis of ceramics is more specifically described in the case of ceramics of the Ca0.25Cu0.75TiO3 family in articles “Polymeric citrate precursor route to the synthesis of the high dielectric constant oxide, CaCu3Ti4O12” by P. Jha et al. published in Material Letters 57 (2003) 2443-2446 and “Synthesis of Ca0.25Cu0.75TiO3 and infrared characterization of role played by copper”, by A. Hassini, M. Gervais et al., published in Materials Science and Engineering B87 (2001) 164-168. This synthesis is performed according to the following method.
The different components, copper, calcium, and titanium, are brought together according to stoichiometric proportions Ca0.25Cu0.75Ti in the form of liquid precursors such as copper nitrate, calcium nitrate, and titanium citrate. The previous precursors are mixed with one or several monomers capable of causing a bi-directional polymerization, such as an acrylamide mixture (added by a proportion of 6% of the total volume) and of N,N′-methylbisacrylamide (added by a proportion of from 3 to 3.5% of the total volume). The polymerization is then performed. During the polymerization, the mixture is heated to a temperature on the order of 100° C. and is generally stirred. Due to the presence of copper, a catalyst such as azoisobutyronitrile is generally introduced to trigger a polymerization. Once the polymerization is over, after from 5 to 30 minutes according to the volume of the solution, an organic gel, sometimes called an “auxiliary” gel, is obtained, in which the cations are trapped, set. A crystallization by a calcinations under ventilation is then performed at a temperature on the order of from 650 to 750° C. for approximately twenty hours. A powder formed of the lamellar aggregation of crystal microstructures in wafers is then obtained. This powder is then shaped as a bar or a pellet, preferably, in the presence of a binder such as polyvinylic alcohol (PVA) or polyethyleneglycol (PEG). For this purpose, it is started by breaking the wafers by a stirring in the presence of zirconia and isopropanol balls, by the removal of the balls, and a drying capable of evaporating the isopropanol. A small amount of binder is added to the crystal powder thus obtained and the mixture is completely dried in a furnace at 200° C. for at least four hours. The resulting dry product is shaped up by isostatic pressing and sintering at 1,000° C. for some twenty hours.
The obtained target is used as a source of the species of a thin layer deposition by bombarding. The dielectric constant of the CaCu3Ti4O12 target is measured and ranges between 700 and 3,000, as discussed in previously-mentioned article “Polymeric citrate precursor route to the synthesis of the high dielectric constant oxide, CaCu3Ti4O1 2” by P. Jha et al.
The obtained limiting value of 3,000 is much smaller than the theoretical value of the dielectric constant of a material of type Ca0.25Cu0.75TiO3 ranging between 9,000 and 12,000.
The dielectric characteristics depend in particular on the density and on the size of the target ceramic grains.